1. Field of the Invention
The present invention relates to a method and semiconductor device for monitoring battery voltages.
2. Description of the Related Art
Semiconductor devices for monitoring a plurality of unit batteries or battery cells connected in series are already available. Well known examples include battery monitoring integrated circuits (ICs) for vehicles. For example, in Japanese Patent Application Publication No. 2003-070179, Miyazaki et al. disclose an electrical storage device including a plurality of high-energy cells connected in series in a hybrid electric vehicle, and a method of controlling the electrical storage device. The disclosed method employs a battery monitoring IC with the configuration shown in simplified form in FIG. 1.
The battery monitoring IC 100 in FIG. 1 is connected through a low-pass filter circuit (LPF) 116 to a battery 114 including five battery cells Vc5 to Vc1 coupled in series. The positive terminal of battery cell Vc5 is connected to the VCC power supply terminal of the battery monitoring IC 100. The negative terminal of battery cell Vc1 is connected to ground (GND). The two terminals of each of the battery cells Vc5 to Vc1 in the battery 114 are connected through the low-pass filter circuit 116 to corresponding inputs of a cell selector switch 118 in the battery monitoring IC 100. The low-pass filter circuit 116 includes resistors Rf5 to Rf0 and capacitors Cf5 to Cf1. The cell selector switch 118 includes switching elements SW5, SW4_1, SW4_2, . . . , SW1_1, SW1_2, SW0, which receive voltages V05, V04, . . . , V02, V01. The outputs Vx, Vy of the cell selector switch 118 are connected to an analog level shifter 122 that includes current sensing resistors R4 to R1, an amplifier 136, and dummy switches SWA and SWB. The dummy switches SWA and SWB are similar to the switching elements in the cell selector switch 118 except that they are permanently switched on. External resistors RD2 and RD1, having the same resistance value as resistors Rf5 to Rf0 in the low-pass filter circuit 116, are connected to the analog level shifter 122 at terminals Vout2 and Vout1 to provide symmetrical paths from the input terminals of the amplifier 136 to the battery 114, the output terminal of the amplifier 136, and ground.
To measure the voltage of battery cell Vc5, for example, switching elements SW5 and SW4_1 are turned on and the other switching elements in the cell selector switch 118 are turned off. Since the voltage-sensing resistors R1-R4 have mutual identical resistance values and switching elements SW5, SW4_1, SWA, and SWB have mutual identical on-resistance values, the voltage (V5−V4) across battery cell Vc5 is converted by the analog level shifter 122 to an equal output voltage Vout with respect to ground (Vout=V5−V4).
The voltages across battery cells Vc4 to Vc1 are measured similarly, with switching elements SW4_2 and SW3_1, SW3_2 and SW2_1, SW2_2 and SW2_1, and SW1_2 and SW0 turned on, respectively.
Referring to FIG. 2, the switching elements in the cell selector switch 118 are metal-oxide-semiconductor (MOS) transistors having respective parasitic capacitances C5, C41, C42, . . . , C11, C12, C0 with respect to ground. The dummy switching elements SWA, SWB and the wiring connecting them to the cell selector switch 118, amplifier 136, and ground also have parasitic capacitances. When the switching elements in the cell selector switch are all turned off, these parasitic capacitances discharge to ground level.
When switching elements SW5 and SW4_1 are switched on to measure the voltage across battery cell Vc5, for example, there is sudden transfer of charge on the paths indicated by the thick black lines in FIG. 2. The voltage V51 received by switching element SW5 plunges as charge moves from capacitor Cf5 in the low-pass filter circuit 116 toward ground through terminal Vout1, and into the parasitic capacitances en route. This plunge is followed by a gradual recovery as capacitor Cf5 is recharged from the positive terminal of battery cell Vc5.
In addition, when switching element SW5 is switched on, the voltage at the non-inverting input terminal of amplifier 136 rises. The output voltage Vout rises abruptly in response and is fed back through terminal Vout2, pulling up the voltage V41 input to switching element SW4_1. Due to the reduced difference between voltages V51 and V41, however, the initial abrupt rise in the output voltage Vout is followed by a droop that persists for some time until the correct output value (V5−V4) is attained. These changes in V51, V41, and Vout are illustrated schematically in FIG. 3.
The time T taken to recover from the output voltage droop depends on the time constant of the low-pass filter circuit 116, which increases with the resistance values of resistors Rf5 to Rf0 and the capacitance values of capacitors Cf5 to Cf1. If the time constant is large enough to provide the filtering effect needed for accurate measurement, the recovery period T can be fairly long.
During the recovery period T and until the measurement ends, there is a continuing flow of current from the battery 114 to ground and to the output terminal of the amplifier 136 on the paths indicated in FIG. 2. For accurate measurement, the transistors in the cell selector switch 118 must be large enough to carry these currents. With the conventional battery monitoring IC 100, accordingly, it can take significant time for the output of the analog level shifter 122 to stabilize and the correct measurement to be obtained, and large switching elements are necessary.